Ablative armor is a type of protective hull plating designed to negate damage by itself being damaged or destroyed through the process of ablation.
Ablative armor was developed by Starfleet in the late 24th century, and was designed to provide an additional layer of defense to starships, which would disintegrate at a controlled rate when under directed energy blasts. (DS9 reference: Star Trek: Deep Space Nine Technical Manual)
One of the first Federation starships to be equipped with ablative armor was the Defiant-class prototype, USS Defiant. Between 2371 and 2375, the ablative armor proved vital in giving the Defiant an added advantage when engaged in conflict with other larger vessels, such as engaging Klingon attack cruisers, and the renegade Excelsior-class vessel, USS Lakota. (DS9 episodes: "The Way of the Warrior", "Paradise Lost")